We have used genetics and molecular biology to characterize endogenous murine leukemia viruses (muLVs) and to identify and map their genes and gene functions. We plan to extend these studies in order to obtain fine structure genetic maps of the viruses. In particular we are interested in sequencing regions of the genome that specify the N-, B-, or NB-tropic host range, and the ecotropic/dual tropic host range of MuLVs. We believe that this detailed analysis of MuLV genomes will, in conjunction with biological studies and analysis of viral proteins obtained in our and others' laboratories, facilitate our understanding of the replication and oncogenic potential of these viruses.